The present disclosure relates to an optical apparatus that performs observations and measurements based on work images.
From the past, optical apparatuses that form images of works by optical systems each including an objective lens and an imaging lens and perform observations and measurements based on the work images have been known. For example, in a measurement projection apparatus, an image of a work is projected by a projection section, and the projected work is visually measured. Also in an image measurement apparatus, an image of a work is projected by a digital camera or the like, and a measurement or the like is performed based on the digital image. In the optical apparatuses as described above, it is important to highly-accurately focus on a work image.
The focusing is performed by relatively moving a stage on which a work is mounted and a measurement head on which an imaging optical system is mounted in a vertical direction, for example. Typically, one of the stage and the measurement head is moved upwardly or downwardly by rotating a rotation handle. By the weight of the stage or the measurement head, a difference may be caused between a rotation torque requisite for the raising and a rotation torque requisite for the lowering. An increase in the difference between the rotation torques requisite for the upward and downward movements induces lowering of operability of the optical apparatus.
Japanese Patent Application Laid-open No. Hei 06-6159 (hereinafter, referred to as Patent Document 1) discloses, though in a totally different field, a brake unit obtained by combining a friction-type brake and a one-way clutch. By using the brake unit, it is possible to equalize the rotation torques requisite for lowering and raising a lifter plate on which an automobile or the like is mounted (see, for example, left column of page 3 in Patent Document 1).